ZyXEL Communications ZyAIR B-5000, Руководство пользователя

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ZyAIR B-5000 User’s Guide
28-4 Firewall
Teardrop program creates a series of IP fragments with overlapping offset fields. When these
fragments are reassembled at the destination, some systems will crash, hang, or reboot.
2. Weaknesses in the TCP/IP specification leave it open to " SYN Flood " and " LAND " attacks. These
attacks are executed during the handshake that initiates a communication session between two
applications.
Figure 28-2 Three-Way Handshake
Under normal circumstances, the application that initiates a session sends a SYN (synchronize) packet to the
receiving server. The receiver sends back an ACK (acknowledgment) packet and its own SYN, and then the
initiator responds with an ACK (acknowledgment). After this handshake, a connection is established.
2-a SYN Attack floods a targeted system with a series of SYN packets. Each packet causes the
targeted system to issue a SYN-ACK response. While the targeted system waits for the ACK that
follows the SYN-ACK, it queues up all outstanding SYN-ACK responses on what is known as a
backlog queue. SYN-ACKs are moved off the queue only when an ACK comes back or when an
internal timer (which is set at relatively long intervals) terminates the three-way handshake. Once
the queue is full, the system will ignore all incoming SYN requests, making the system unavailable
for legitimate users.